Fix func to determine default login (#29)

only add default login if auth profided or no login entry exist

Reviewed-on: https://codeberg.org/woodpecker-plugins/plugin-docker-buildx/pulls/29
Reviewed-by: qwerty287 <qwerty287@noreply.codeberg.org>

cmd/docker-buildx/config.go

@@ -117,7 +117,7 @@ func settingsFlags(settings *plugin.Settings) []cli.Flag {
 			Name:        "tags",
 			EnvVars:     []string{"PLUGIN_TAG", "PLUGIN_TAGS"},
 			Usage:       "sets repository tags to use for the image",
-			Value:       cli.NewStringSlice([]string{"latest"}...),
+			Value:       cli.NewStringSlice("latest"),
 			FilePath:    ".tags",
 			Destination: &settings.Build.Tags,
 		},